It is a problem, it teaches her the wrong lesson that when adults tell you secrets you can't tell anyone ect. I guess with working with children and doing child abuse courses ect I see it differently ?
Disclaimer. This is off topic so warn me, suspend me, ban me at your will but I cannot in all conscience keep my mouth shut about this ridiculous and potentially damaging comment.
There is absolutely NO comparison between a mother telling her child to keep a secret for a couple of hours about a family treat and keeping a secret of the kind you learn about doing child abuse courses. It is absolutely sickening that you would even try to make one.
I've done multiple courses in Child Protection. I was a Special Needs Teacher for many years and now work in as a Hospital Teacher. I've also had practical experience of being a child's advocate in situations where that child was being abused and as you should well know if you'd paid attention in those courses, the situations they deal with are not a matter for stupid emoticons, cheap point scoring or trying to claim the moral high ground.
Moreover IF a child in my care was upset about a minor issue (and to be clear I'm talking about something like their hamster having died or their brother having shaved the hair off their favourite Barbie) I would have absolutely no problem telling them about a planned treat for the whole class to take place later that day and asking them to keep it as a surprise IF I felt it would help the child and distract them from their immediate worries.